The purpose is to determine the mechanisms responsible for the increased susceptibility to and impaired recovery from contraction-induced injury of skeletal muscles in old animals compared with those in younger animals, and for the protection from contraction-induced injury provided by conditioning programs of pliometric (lengthening) contractions. Contraction-induced injury provided by conditioning programs of activated muscle fibers are stretched as opposed to shortened or held at fixed length. The magnitude of a contraction-induced injury is best predicted (r2 = 0.80) by the product of the length of the stretch and the concurrent average force developed. The injury is initiated by the mechanical disruption of the ultrastructure of small groups of sarcomeres. After repeated exposures to moderate protocols of pliometric contractions, muscles in animals of all ages become "conditioned" and are no longer injured by protocols of pliometric contractions which previously caused injury. The working hypotheses are that: (i) contraction-induced injury occurs when weaker sarcomeres in series with stronger sarcomeres are stretched beyond overlap of thick and thin filaments and are damaged; and (ii) pliometric conditioning protects muscle fibers from contraction-induced injury by initiating injury to the weaker sarcomeres with subsequent replacement of weaker by stronger sarcomeres. Hypotheses will be tested through in vivo and in situ experiments on hind-limb muscles of specific pathogen free young (3 month), adult (12 month), and old (26 month) mice and in vitro experiments on single permeabilized fiber segments and single intact muscle fibers from mice, rats, and frogs. The clarification of the role of contraction-induced injury in the development of muscle atrophy and weakness that occurs with aging is of the utmost importance. Understanding the mechanisms responsible for the increased susceptibility to and impaired recovery from contraction-induced injury for muscles of old animals and for the protection provided to muscles by pliometric conditioning will facilitate the design of conditioning programs to protect muscles, particularly muscles of old people, from injury.